Cellulose nitrate product



about 0.25 to 0.5.

United States Patent O CELLULOSE NITRATE PRODUCT Arthur SloamWashington,D.C., and David J. Mann,

Wharton, N.J., assignors to Atlantic Research Corporation, Alexandria,Va, a corporation of Virginia No Drawing. Application August 13, 1954Serial No. 449,775

2 Claims. (or. 260-223) This invention relates to a new and improvedcellulose nitrate product in the form of small, spherical particles ofhigh density.

Processes for preparing ball powder from high nitrogen content cellulosenitrate (12.4-13.4% nitrogen) are described in Olsen et al., U.S.2,027,114 and Shaefer, US. 2,160,626. In the former patent, a colloid,such as starch, is used to obtain' sphere formation and the products arehighly porous, with specific gravities ranging from In the latterpatent, a solute such as sodium sulfate, is added to the aqueous phaseof the emulsion to increase density of the cellulose nitrate spheres. Itis stated that specific gravities as high as 0.8 to 1.0 may be obtained.However, in view of actual specific gravity of high nitrogen contentcellulose nitrate of 1.58 to 1.66 depending upon degree of nitration, itis evident that a high nitrogen product having a specific gravity of 0.8to 1.0 possesses a high degree of porosity. For some uses, considerablydenser spherical particles are required. An additional disadvantage ofthese processes stems from the fact that the smallest particle producedis about 6 mils or 150 microns and these are obtainable only as ascreened fraction of a batch containing a large proportion ofconsiderably larger spheres. Thus these processes cannot meet arequirement for cellulose nitrate spheres smaller than 6 mils and eventhe smaller sizes produced by these methods constitute only a portion ofa given production.

The object of this invention is to provide small, dense, sphericalparticles of cellulose nitrate of relatively low nitrogen content,namely cellulose nitrate containing up to 12% nitrogen.

Processes for making such small, dense, spherical particles of cellulosenitrate are described and claimed in Sloan and Mann patent applicationSerial No. 449,776, filed concurrently herewith, now Patent No.2,891,054, granted June 16, 1959.

As disclosed in said Sloan and Mann application, particles having thedesired characteristics of size, sphericity and high density may beobtained when the cellulose nitrate in specified concentration isdissolved in an organic solvent characterized by certain essentialproperties and the resulting lacquer is dispersed with vigorousagitation in specified amounts of water in the presence of certaincolloids together with water-soluble salts or water-soluble polyhydroxycompounds to form an oil-in-water emulsion. In some cases it is alsodesirable to introduce a polar-type, surface-active emulsifying agent.

The cellulose nitrate solvent may be a single solvent which ismoderately soluble in water, a mixture of solvents, the components ofwhich are moderately soluble in water, or a mixture of solvents, atleast one component of which is moderately soluble in water and anothercomponent of which is infinitely soluble in water. By moderately solubleis meant a solvent which is soluble in water at least to the extent ofparts per 100 parts of water at 20 C. but is not infinitely soluble. Byinr 2,931,860 Patented Apr. 5, 19cc finitely soluble is meant a solventwhich is soluble in water in all proportions. Maximum solubility inwater of the cellulose nitrate solvent should be about parts per partsof water and preferably about 40 to 50 parts. Thus, if a mixed solventis employed containing an infinitely soluble component, the amount ofinfinitely soluble component must be adjusted to maintain totalsolubility in water of the mixed solvent within the desired range.

Examples of moderately water-soluble organic solvents include methylacetate, ethyl acetate, methyl formate, ethyl formate, methyl ethylketone, diethyl ether and nitromethane. The moderately water-solublesolvents preferably range in water-solubility from about 5 to 30 partsper 100 parts of water at 20 C.

Infinitely water-soluble co-solvents include the loweraliphatic alcoholssuch as methyl, ethyl and propyl alcohols, acetone, methyl lactate,ethyl lactate, and dioxan-l,4.

Where co-solvent mixtures are used, it is not essential that each of thesolvent components be a good solvent for the cellulose nitrate so longas they possess good solvent properties when in admixture. Diethyl etherand the lower aliphatic alcohols, for example, though poor solvents forcellulose nitrate per se, possess excellent co-sol-. vent properties. Itis desirable that the infinitely soluble component of a mixed cellulosenitrate solvent possess the ability to reduce viscosity of the lacquer.Methanol, ethanol and acetone are particularly effective in this regard.Y

We have found that particles having the desired ch'arac teristics cannotbe obtained with a cellulose nitrate solvent which is substantiallyinsoluble in water, namely soluble in water to the extent of less thanabout 5 parts per 100, or which is excessively soluble. The watersolubility must be sufficient so that when the lacquer particles aredispersed in water, the solvent at and adjacent to the surface dissolveswith sufiicient rapidity in the water to permit some hardening orsetting of the particle surface and thus eliminates or markedly reducesthe surface tackiness to the point .where such surface tackiness willnot cause agglomeration. On the other hand, elution of the solventshould not be so rapid that the surface sets into a hard, non-plasticcondition before surface tension forces can effectively shape theparticles into the desired spherical form. Premature excessivelyhardening of the particle surface also causes porosity since thenon-plastic surface does not permit adequate shrinkage to compensate forremoval of solvent from the interior of the particle. With a cellulosenitrate solvent of excessive water-solubility, porosity may also becaused by migration of water into the particle because of the mutualsolubility of water and a highly water-soluble solvent. Asaforementioned, the water-solubility of the cellulose nitrate solvent,whether "ice single or mixed, should be within a range of about 5 to 80parts per 100 parts of water at 20 C. and preferably about 5 to 40 or50.

We have found that to obtain spheres of the desired small size,concentration of the low nitrogen content cellulose nitrate in thelacquer should not exceed about 20% by Weight. Optimum concentration isabout 8 to 16%. Lacquers containing more than about 20% of thelownitrogen cellulose nitrate are highly viscous and are comminuted withdifficulty to produce particles of generally excessive size. lulosenitrate concentration in the lacquer other than that dictated byeconomic expediency. Good results are obtainable, for example, withconcentrations as low as 2%. This, however, requires the use of largeamounts of solof cellulose nitrate processed.

There is no critical lower limit of ce1- Choice of the particularsolvent influences to some degree the cellulose nitrate concentration inthe lacquer. Viscosity of the lacquer varies with different solvents andthe concentration of cellulose nitrate may be varied accordingly. a

Solvent systems which we have found to be particularly satisfactoryinclude methyl acetate, methyl acetate/ methanol, ethyl acetate, ethylacetate/ ethanol, ethyl acetate/ acetone, ethyl acetate/diethyl ether,methyl ethyl ketone/acetone, methyl ethyl ketone/methanol andnitromethane.

Where a mixed cellulose nitrate solvent as, for example, one containingan infinitely soluble component, is employed, the ratio of componentsfor optimum performance can readily be determined by routineexperimentation. In most cases a ratio of moderately soluble componentto infinitely soluble component of 80:20 to 85:15 by volume isparticularly good. However, this may be varied so long as totalsolubility of the mixed solvent does not become excessive.

Choice of solvent is also influenced to some extent by the particularcolloiding agent employed. The protective colloid must not beexcessively soluble in the cellulose nitrate solvent since, otherwise,the system would tend to form a water-in-oil type emulsion. Thissituation can be handled by employing a colloid which is not highlysoluble in the specific solvent or by avoiding the use of solvents whichtend to dissolve the particular colloid out of the water phase.

The cellulose nitrate lacquer is mixed with water in the presence of asuitable colloiding agent and a watersoluble salt or polyhydroxycompound, with vigorous agitation, to form a dispersion of smallspherical lacquer particles with the water as the continuous phase. Thecolloid and salt are preferably introduced into the water prior tomixing. The dispersion and agitation may be accomplished by any suitablemeans as, for example, in a homogenizer or other suitable agitatingdevice. The degree of agitation is one of the factors influencingparticle size. In general, the more vigorous the agitation, the smallerare the particles.

The amount of water employed in dispersing the cellulose nitrate lacqueris an important factor in determining particle size. Particle size is afunction of the lacquer/ water ratio in as much as increasing the-amountof water increases particle size. Thus the quantity of water used inemulsification is determined in some measure by the particle sizedesired. The minimum amount of water is governed by the lacquer/waterratio required to permit formation of the requisite oil-in-water typeemulsion. If excessive water is employed, the particles become oversizeand may also become irregular and porous, apparently because the organicsolvent dissolves in the water phase with excessive rapidity so that thesurface of the particles hardens into a non-plastic condition beforeadequate comminution or contraction into spheres. In general, themaximum amount of water which gives satisfactory results in terms ofdesired particle characteristics is about 100% by volume based on thevolume of organic solvent and preferably about 50 to 75%.

We have found that particle characteristics such as size and density aregreatly influenced by the particular colloid employed. Dense particleswithin the desired size range can be obtained with the followingcolloiding agents: methyl cellulose, casein, the condensation prodnet ofpolyvinyl methyl ether and maleic anhydride, gelatin, agar-agar,polysodium acrylate, polyvinyl alcohol and alginates.

The colloiding agent must be preferentially soluble in water rather thanin the organic solvent to avoid formation of a water-in-oil system.Although the aforementioned colloids are normally hydrophilic, certainones may be excessively soluble in a particular organic solvent. Methylcellulose, for example, cannot be employed with ethyl acetate because ofits high solubility the like.

in this solvent, although it functions well with other solvents such asmethyl ethyl ketone/acetone. It is essential, therefore, to employorganic solvent-colloid systems of low mutual solubility relative to thesolubility of the colloid in the water. The nitromethane-polyvinylmethyl ether-maleic anhydride system, for example, is excellent becauseof the insolubility of the colloid in this solvent.

The colloiding agent forms a colloidal solution with the water, thusincreasing the viscosity of the water phase, and also surrounds thelacquer particles with a protective coating. These factors stabilize theemulsion by reducing tendency of the particles to coalesce, particularlyduring stripping of the solvent. This is important since the particlesmust be maintained in a state of dispersion until surface tackiness hasbeen substantially eliminated and hardening has progressed to the pointWhere the particles will no longer agglomerate.

The amount of colloiding agent employed is determined by severalfactors. Primarily, it should be sufficient to produce substantialincrease in the viscosity of the water phase and to coat the particles,thus providing adequate stabilization of the emulsion during processing.Generally speaking, the higher the molecular weight of the colloid, theless will be required. The colloid is also a factor in controllingparticle size since increasing concentration tends to reduce particlesize. There is no critical upper limit to the amount of colloid employedother than practical considerations such as desired partiele size andease of washing. The more colloid present, the more washing is requiredfor its removal.

Colloid-organic solvent systems which we have found particularlysuitable for our purpose include casein plus ethyl acetate, methylcellulose plus methyl acetate, methyl ethyl ketone/acetone or methylethyl ketone/methanol, polyvinyl methyl ether-maleic anhydride plusnitromethane or ethyl acetate, alginates plus ethyl acetate and gelatinplus ethyl acetate. The methyl cellulose is preferably of the highermolecular weight variety, namely one having a minimum centipoise valueof about 400 and preferably about 1500 to 4000 or higher.

It is essential that a siutable water-soluble salt or an organicpolyhydroxy compound be included in the water phase of the emulsionsystem. It is apparently necessary for the proper functioning of theprotective colloid. Without the addition of such compounds, theparticles formed are non-spherical and oversize. The salt or polyhydroxycompound prevents the inversion of phase which frequently occurs whenthe emulsion is heated to distillation temperature to remove the organicsolvent. It may be, also, that these additives exert a stabilizingeffect by causing hydration of the colloid and thus preventing unduepenetration of the colloid into the lacquer particle, and, in the caseof the salts, by inducing a charge on the surface of thecolloid-enveloped particle which tends to prevent agglomeration.

Any suitable, water-soluble salt which is compatible with the colloidingagent, namely one which will not cause it to precipitate, may beemployed as, for example, the metal or ammonium halides, sulfates,acetates and Choice of the particular salt employed is, in some measure,determined by the particular colloid used in the emulsion system. All ofthe colloiding agents except methyl cellulose function will with saltsof any valence, preferably monovalent or divalent salts, as for example,sodium chloride or sodium sulfate. Methyl cellulose, however, toleratesonly monovalent salts such as the alkali metal and ammonium halides. Bymonovalent salt is meant one in which both the cation and anion aremonovalent. By divalent salt is meant one in which at least one ion isdivalent. In general, inorganic salts having monovalent cations arepreferred, the alkali metal salts being most desirable.

The salt must be employed in an amount less than that which will c seprecipitation of the colloid or a salting out effect. In general, thelower the molecular weight of the colloid, the greater the amountof'salt which it will tolerate; Degree of salt toleration also varieswith the particular salt.

We have found that below certain minimum salt concentrations the salt isnot effective since the resulting particles tend to be oversize andnon-spherical. This minimum amount varies with the particular salt. Ingeneral, the minimum concentration for the monovalent salts is about6.5% based on the weight of the cellulose ester and for divalent salts,such as sodium sulfate, about 13%.

Additives other than salts which we have found to perform satisfactorilyare water-soluble polyhydroxy compounds such as sugars, glycerin andglycols such as ethylene glycol. In general, a minimum of about 6.5

on the cellulose nitrate is desirable for satisfactory performance.

In some cases, itmay be desirable, though not essential, to include inthe system a polar-type, surface-active emulsifying agent which producesa marked lowering of interfacial tension and which, unlike the highermolecular weight colloiding agent, does not produce any appreciableincrease in the viscosity of the water phase. The surfaceactiveemulsifying agent is advantageous in as much as in some instances itmakes possible a reduction in the amount of colloid required for optimumperformance and thus facilitates subsequent removal of the colloid fromthe particles. It also reduces the amount of energy and time required todisperse the lacquer in the water and to comminute it to the desiredparticle size.

Any suitable surface-active emulsifying agent may be employed which iscompatible with the other components of the system and which issubstantially soluble in water, namely possesses a sufiiciently highhydrophile-lipophile balance to prevent its being drawn into the organicsolvent and converting the emulsion into a water-in-oil system, such asalkyl sulfates or sulfonates, alkylaryl sulfonates, alkali metal soaps,alkali metal and ammonium salts of perfiuoro acids, alkali metal saltsof sulfosuccinic acids, sulfonated oils including sulfonated vegetableoils and sulfonated hydrocarbon oils, polyglycols and the like. 1

The moderately active types of emulsifying agents such as the sulfonatedvegetable oils as, for example, sulfonated castor oil, sulfonatedcoconut oil and the like, sulfonated hydrocarbon oils as,-for examplesulfonated petroleum fractions, alkali metal fatty acid soaps,polyglycols such as polyethylene glycol and polypropylene glycol, andthe like, are especially satisfactory. The polyglycols, in addition totheir dispersing action, also may be advantageous as coupling agents,namely as agents which increase solubility of the colloid or the organicsolvent in water.

The amount of surface-active emulsifier is not critical but should besufficient to promote the desired rapid emulsification and comminutionof the particles. As little as about 0.01 to 0.025% based on the waterphase may be adequate. The amount may be increased to as much as S to10% in some cases. Concentrations of emusifier in the range of about0.01 to 2% are generally satisfactory. The emulsifier may be added tothe water or to the lacquer phase.

After emulsification is completed, the organic solvent is removed fromthe dispersed particles by distillation or by elution. In either case,the emulsion should be maintained in a state of vigorous agitation.

Distillation is accomplished by heating the emulsion to or near theboilingpoint of the organic solvent. If the distillation is conducted atatmospheric pressure, it is desirable that the solvent or the leastvolatile component of a mixed solvent has a boiling point below 100 C.to maintain stability both of the cellulose nitrate and of the emulsion.In many cases, it may be desirable to distil under reduced pressures,particularly if the boiling point is above 100 C.

Another efiective method for removing the organic solvent is by dilutingthe emulsion with water in amount sutficient substantially completely todissolve thesolvent out of the cellulose nitrate particles. Since it isdesirable to maintain the effective salt' or polyhydroxy compoundconcentration throughout the disperse phase of the particles, itispreferable to include these solutes in the elutionwater prior todilution of the emulsion. The total amount of water should be in excessof the theoretical amount required for solution of the organic solvent,preferably in substantial excess.

Afterremoval of the organic solvent, the cellulose nitrate particles areseparated from the water, washed with water and dried. Removal of thecolloid may require several washings. In general, the smaller theparticles, the more water washings are required.

The cellulose nitrate particles prepared according to our process arespherical and may be obtained in sizes by number be such that themaximum average size is about 50 microns. Since all of the particles ina given production batch are obtainable in the desired small-size range,there is no necessity for fractional screening or reworking ofexcessively large particles.

Density of the cellulose nitrate spheres is high. Minimum averagedensity of the low-nitrogen-content cellulose nitrate particles obtainedby the foregoing procedures is about 1.1 to 1.2.

The small, dense, spherical particles of cellulose nitrate are highlysuitable for use in the manufacture of lacquers and plastics in anydesired and conventional manner. They are especially useful, however, inthat they can be suspended in a non-volatile plasticizer to formhomogeneous, stable, fluid slurries which may be poured as coatings orfilms without requiring the addition of water or a volatile solvent andmay be molded into objects of any desired shape and size without theappliaction of high temperatures and pressures.

It may be desirable toincorporate stabilizingagents for the cellulosenitrate such as diphenyl amine, lecithin} ethyl centralite and the like.These are readily introduced by addition to the organic solvent prior tosolution of the cellulose nitrate or to the cellulose nitrate-solventsolution.

EXAMPLE I 30 grams of cellulose nitrate (10.51l.5% N) were dissolved in200 ml. ethyl acetate to form a clear lacquer. 8 grams casein, 8 gramsNa SO and 0.3 gram ethyl centralite were dissolved in ml. water. Theaqueous solution was added to the cellulose nitrate lacquer in ahomogenizer. The emulsion was agitated vigorously for about 5 minutesand then was heated to about 77 C. to distil off the ethyl acetate fromthe dispersed spherical particles. Vigorous agitation was maintainedduring distillation. The product was washed with water 3 times,filtered, washed again with water and dried. The product was in the formof spherical particles ranging in size from 2 to 20 microns and having adensity of 1.2.

EXAMPLE H ml. water. The aqueous solution was added to the cellulosenitrate solution in a homogenizer. The emulsion was agitated vigorouslyfor about 5 minutes. The organic solvent was dissolved out of thespherical partigles by flooding the emulsion with about 4 volumes orwater containing about 2% of the salt with continued agitation. Theproduct was filtered, washed with water 3 times and then dried. Thespherical particles ranged in. size from 1 to microns and had a densityof about 1.11. v V

by their abi i y o be i perseclin pla tiei crto torn stable, fluid suspn 2. Cellulose nitrate, containing up to 12% nitrogen. in the form ofsubstantially spherical, dense particles, having a maximum diameter ofabout 30 microns and a minimum density of 1.1, said particles beingcharacter- Other examples illustrating our invention are sumized bytheir ability to be dispersed in a plasticizer to marized in Table I.The particles in all cases were form stable, fluid suspensions. sherical.

Table I Cellulose nitrate Solvent, ml. Colloid, grams Other additivesSolute, grams Wat e Size, n

, m PercentN g.

Ethyl acetate, s00 Casein, Ethyl centralite, 0.6g Nagsoh 20.... 3751-10. do Casein, .-do Na1SO4, 375 2-20.

Ethylacetate,2nn Casein,$i Ethyl centralite, 0.3g NaOl,4 150 2-20, av10. do do d NaCl, 2 150 2-20, av. 8 --..do. do. 150 2-30, 15 do Gelatin,8. 150 14; .do.. do 01,4 150 1-3D,a s

Ethyl acetate, 720 Ge1ati}1,28.8 do 640 1-6. Ethyl acetate, 170; EthanolPolyvinyl methyl ether- Marasperse 031.5 g Na;SO ,8.-... 150 5-50 30.maleic anhydride, 2. Methyl ethyl ketone, 170; Methyl cellulose, 4000cps., Ethyl centralite, 0.3 g.; NaCl, 4 150 2.

Methanol, 30. Turkey red 011, 1.6 g. do Methyl cellulose, 4000 cps, .-doNaCl,-l 150' 2-40. 10.61l.5 3o ..do M thyl cellulose, 4000 cps, ......doNaQLt 150 1-20. 105-115,-.- so Methylascetate,164;Meth- M ethylcellul0se,4000 cps.. do NaCl,4 150 1-15.

8110 ns-11.5,... M thil etlyl ketone, 170; M etahyl cellulose, 4000 cps,do Na0l,4 150 1-2.

no 0118 0. Methyl ethyl ketone, 200.-.- Gelatin,8 Ethyl centralite, 0.3g- NaCl,4 150 1-25. Nitromethane,200- Alginate, 4". dn NaOL-t 150 1-50.Ethyl acetate, 200.-.. Agar agar,4 d0 NaCl,4 150, l-20. Nitromethane,200. do do- NaCl,4 150 l-25.

1 Calcium lignosnlfonate.

I Kelcolold LV.

Although this invention has been described with refer- References Citedin the file of this patent e'nce to illustrative embodiments thereof, itwill be ap- UNITED STATES PATENTS parent to those skilled in the artthat it may be embodied I in other forms but within the scope of theappended 40 2,027,114 Olsen et 7, 1936 2,160,626 Schaefer May 30, 1939claim, 2,213,255 Olsen et al. Sept. 3, 1940 2,375,175 Silk May 1,1945 1. Cellulose nitrate, containing up to 12% nitrogen, 111 2,715,574COX '16 1955 the form of substantlally spherical, dense particles havinga. maximum average diameter of about microns, a maximum diameter ofabout microns and a minimum density of 1.1, said particles beingcharacterized OTHER REFERENCES

1. CELLULOSE NITRATE, CONTAINING UP TO 12% NITROGEN, IN THE FORM OFSUBSTANTIALLY SPHERICAL, DENSE PARTICLES HAVING A MAXIMUM AVERAGEDIAMETER OF ABOUT 50 MICRONS, A MAXIMUM DIAMETER OF ABOUT 100 MICRONSAND A MINIMUM DENSITY OF 1.1, SAID PARTICLES BEING CHARACTERIZED BYTHEIR ABILITY TO BE DISPERSED IN A PLASTICIZER TO FORM STABLE, FLUIDSUSPENSIONS.